The invention relates to a method for operating an exhaust gas purification system of an internal combustion engine that can be operated in a lean operating mode with excess air and in a rich operating mode with a lack of air.
EP 0 878 609 A2 describes an internal combustion engine, which can be operated in a lean operating triode with excess air and in a rich operating mode with a lack of air. In one embodiment, the internal combustion engine has an exhaust gas purification system in which an ammonia-forming catalyst, an ammonia-SCR catalyst baying a storage capacity for ammonia (NH3) and a nitrogen oxide storage catalyst are arranged one after the other in the direction of flow of the exhaust gas. The nitrogen oxide storage catalyst has the property of being able to store nitrogen oxides (NOx) contained in the exhaust gas under lean operating conditions of the internal combustion engine. Under rich operating conditions, nitrogen oxides (NOx) stored in the NOx storage catalyst are reduced by active reducing components present in the exhaust gas, thereby regenerating the NOx storage catalyst. In addition, NOx present in the exhaust gas is reduced at least partially to NH3 by reaction with the active reducing exhaust gas constituents on the NH3-forming catalyst under rich operating conditions, and the NH3 thereby formed is stored in the downstream NH3-SCR catalyst. If the internal combustion engine is operated in the lean operating mode, then NOx present in the exhaust gas can be removed from the exhaust gas by reducing it with NH3 stored in the NH3-SCR catalyst as well as by storage in the NOx storage catalyst. In comparison with exclusive use of a NOx storage catalyst, the decrease in NOx content can be improved in this way.
The object of the invention is to provide a method for operating an exhaust gas purification system of the type defined in the introduction, which will permit the most thorough possible removal of pollutants from the internal combustion engine exhaust gas.
The method according to the invention is used in an exhaust gas purification system for an internal combustion engine that can be operated in a lean operating mode with an air excess and in a rich operating mode with a lack of air. The exhaust gas purification system has a NH3-forming catalyst, which can catalyze at least partial reduction of NOx contained in the exhaust gas to NH3, a first exhaust gas sensor, a NH3-SCR catalyst with a storage capacity for NH3, a nitrogen oxide storage catalyst directly downstream from the NH3-SCR catalyst, having a storage capacity for oxygen (O2) and a storage capacity for NOx and a second exhaust gas sensor, these units being arranged one after the other in the direction of flow of the exhaust gas. Exhaust gas sensors capable of emitting a first signal that correlates with a NOx content of the exhaust gas and a second signal that correlates with a lambda value of the exhaust gas are used as the first and second exhaust gas sensors. A lambda value is understood to refer to the ratio of the air supplied to the internal combustion engine and fuel (engine lambda value) with respect to the stoichiometric lambda value of 1.0 and/or the oxidation potential and/or reduction potential of the resulting exhaust gas (exhaust gas lambda value), as is customary. In a diagnostic operation in operating phases of the internal combustion engine with lean operating mode and with rich operating mode that follow one another directly, the NH3 storage capacity of the NH3-SCR catalyst, the oxygen storage capacity (OSC=oxygen storage capacity) of the NOx storage catalyst and optionally the NOx storage capacity of the NOx storage catalyst are determined by analyzing the signals of the first and second exhaust gas sensors. The storage of O2, NOx and/or NH3 in the catalysts is to be understood as reversible storage. In other words, stored O2, NOx and/or NH3 can be removed from the catalysts by reaction with the respective exhaust gas components. However, NH3 stored in the NH3-SCR catalyst and NOx stored in the NOx storage catalyst can typically also be used otherwise, for example, by being thermally desorbed.
The inventors have recognized that to achieve a comprehensive, effective and thorough exhaust gas purification, it is necessary to coordinate the duration of the phases of operation of the internal combustion engine with a lean operating mode and a rich operating mode with the storage properties of the NOx storage catalyst and the NH3-SCR catalyst during alternating operation of the internal combustion engine in the lean operating mode and in the rich operating mode, as is carried out during normal operation. If this is not adequately coordinated, unwanted residual emissions of NOx, HC, CO and NH3 may occur. The inventors of the present invention have recognized in particular that inadequate coordination of the rich operating phase with the NH3 storage capacity of the NH3-SCR catalyst can result in unwanted secondary emissions of nitrous oxide (N2O) pollutant. In other words, if a large amount of NH3 is formed due to a prolonged rich operating phase and is stored in the NH3-SCR catalyst, it may result in leakage of NH3 out of the NH3-SCR catalyst. However, NH3 introduced into the NOx storage catalyst can then be oxidized to N2O in the NOx storage catalyst. Because of the determination of the NH3 storage capacity of the NH3-SCR catalyst according to the invention as well as the determination of OSC and optionally the NOx storage capacity of the NOx storage catalyst, this permits optimum adaptation of the duration of the rich operating phase and the lean operating phase to the storage properties of the NH3-SCR catalyst and the NOx storage catalyst and thus at least extensive avoidance of the aforementioned secondary emissions.
It has proven to be particularly advantageous to use exhaust gas sensors between the NH3-forming catalyst and the NH3-SCR catalyst and/or downstream from the NOx storage catalyst for determining the storage properties of the NH3-SCR catalyst and the NOx storage catalyst, each of these sensors being able to emit at least one first signal that correlates with a NOx content of the exhaust gas and one second signal that correlates with a lambda value of the exhaust gas. The exhaust gas sensors have measurement cells which can be reached by the exhaust gas and in which the corresponding signals can be generated. Integrated exhaust gas sensors are preferably sensors which can emit a signal correlating with the NH3 content and/or with the NOx content of the exhaust gas and also a signal correlating with the lambda value of the exhaust gas. However, sensors having a selective sensitivity can also be used for each of the exhaust gas components individually for the first and/or second exhaust gas sensors, but this is not preferred because of the increased equipment complexity. The duration of the respective phase in the lean operating mode and in the rich operating mode is controlled by means of the exhaust gas sensors between the NH3-forming catalyst and the NH3-SCR catalyst as well as downstream from the NOx storage catalyst. To do so, the output signals of the exhaust gas sensors are evaluated by an electronic control unit.
A diagnostic operation with determination of the storage properties of the catalysts, which is typically carried out from time to time, is particularly advantageous because the storage capacity values of the catalysts may undergo changes due to age. If the prevailing storage capacity values have been determined, then the periods of time of the respective phases can be adapted to storage capacity characteristic values of the catalysts, which may optionally have been adapted in the sense of an optimal exhaust gas purification, during the normal alternation of operating phases in the lean operating mode and the rich operating mode. In particular to prevent the formation of nitrous oxide on the NOx storage catalyst, it is advantageous to know the prevailing NH3 storage capacity of the NH3-SCR catalyst and to take it into account because nitrous oxide cannot be detected at all or at least not reliably by means of the second exhaust gas sensor. The duration of operation of the internal combustion engine in the rich operating mode with formation of NH3 on the NH3-forming catalyst can then be determined as a function of the extent of the utilization of the prevailing NH3 storage capacity of the NH3-SCR catalyst.
As has been found, only the OSC of the NOx storage catalyst is a directly determinable storage capacity characteristic value for the serial combination of NH3-SCR catalyst and nitrogen oxide storage catalyst arranged directly following one another when no additional exhaust gas sensor is used between the NH3-SCR catalyst and the NOx storage catalyst, as is preferably provided in the present case. This is due to the fact that the NOx storage catalyst has an OSC but the NH3-SCR catalyst does not. On the other hand, the NH3 storage capacity of the NH3-SCR catalyst and the NOx storage capacity of the NOx storage catalyst cannot be determined directly in such an embodiment of the exhaust gas purification system. This is in turn due to the fact that in general it is impossible to differentiate whether NOx has been withdrawn from the exhaust gas due to reduction with NH3 stored in the NH3-SCR catalyst or due to storage in the NOx storage material of the NOx storage catalyst. A total value can also be determined for the OSC and the NOx storage capacity of the NOx storage catalyst.
In one embodiment of the invention, it is therefore provided that a first total value for the storage capacity for the OSC and NOx storage capacity of the NOx storage catalyst and a second total value for the NOx storage capacity of the NOx storage catalyst and the NH3 storage capacity of the NH3-SCR catalyst are all determined during diagnostic operation, and the NH3 storage capacity of the NH3-SCR catalyst is determined by calculating the OSC thus determined and the first and second total values for the storage capacity.
OSC and NOx storage capacity indicate how many moles or which mass of O2 and/or NOx can be stored in the respective storage material of the NOx storage catalyst under the respective conditions. Since half a mole of O2 and one mole of NOx are necessary to oxidize one mole of CO or H2, OSC and the first total value for the storage capacity can be scaled in CO or H2 equivalents, for example. The first total value for the storage capacity in this case is indicated by the sum of the CO or H2 equivalence of the OSC and the NOx storage capacity of the NOx storage catalyst. The NH3 and NOx storage capacity as well as the second storage capacity total value can each be indicated in N2 equivalence, for example.
It is also provided in another embodiment of the invention that to determine the NOx storage capacity of the NOx storage catalyst in diagnostic operation, a first total value for the storage capacity is determined for the OSC and NOx storage capacity of the NOx storage catalyst and the NOx storage capacity of the NOx storage catalyst is determined by calculating the OSC thereby determined with the first total value for the storage capacity.
For direct determination of the OSC and NOx storage catalyst, in another embodiment of the invention, the internal combustion engine is operated in diagnostic operation in the rich operation mode in a first method step until the NH3 storage capacity of the NH3-SCR catalyst has been at least largely depleted by uptake of NH3 formed in rich operating mode on the NH3-forming catalyst and introduced into the NH3-SCR catalyst with the exhaust gas, and the NOx storage catalyst is at least largely freed of (reversibly) stored NOx and O2 and in a second method step, which follows this directly the internal combustion engine operation is switched to lean operating mode and the OSC of the NOx storage catalyst is determined from the time lag between the second signal of the first exhaust gas sensor which indicates the change from rich to lean operation and the second signal of the second exhaust gas sensor indicating a change from rich to lean operation.
In another embodiment of the invention, to determine the second storage capacity total value, the internal combustion engine is operated in the rich operating mode in a first method step in diagnostic operation until the NH3 storage capacity of the NH3-SCR, catalyst is at least largely depleted by uptake of NH3 formed on the NH3-forming catalyst during the rich operating mode and introduced into the NH3-SCR catalyst together with the exhaust gas, and the NOx storage catalyst is at least largely freed of (reversibly) stored NOx and O2 and in a second method step, which follows the first one directly, there is a change in the operation of the internal combustion engine from the rich operating mode to the lean operating mode, and in a third method step, the internal combustion engine is operated in the lean operating mode until the NOx storage capacity of the NOx storage catalyst has been at least largely depleted by the storage of NOx introduced into the NOx storage catalyst together with the exhaust gas. Then the second storage capacity total value is determined from the first signals of the first and second exhaust gas sensors integrated over the duration of the third method step.
To determine the first total value for the storage capacity, in another embodiment of the invention, starting from lean operation of the internal combustion engine with the NOx storage capacity of the NOx storage catalyst at least approximately depleted, the operation of the internal combustion engine is switched to the rich operating mode and the rich operating mode is maintained until the second signal of the second exhaust gas sensor indicates a switch from lean to rich. The first total value for the storage capacity is then determined from the time lag between the second signal of the first exhaust gas sensor indicating the switch from lean to rich operation and the second signal of the second exhaust gas sensor indicating the switch from lean to rich operation.
In another embodiment of the method, to determine the first total value for the storage capacity, a release of NOx desorbed in unreduced form from the NOx storage catalyst, this release taking place in conjunction with the switch in operation of the internal combustion engine from the lean operating mode to the rich operating mode, is taken into account by summary detection of the first signal of the second exhaust gas sensor, in particular when the NOx storage capacity of the NOx storage catalyst is depleted to a relatively great extent due to the long-term operation of the internal combustion engine in the lean operating mode. For example, desorption of stored NOx may occur when there is a switch to the rich operating mode, wherein the desorbed nitrogen oxides are discharged from the NOx storage catalyst in unreduced form and therefore without consumption of reducing agent. The consumption of reducing agent for regeneration of the NOx storage catalyst is thus reduced, which is why the period of time until the second signal of the second exhaust gas sensor indicates a switch from lean operation to rich operation is reduced. In such a case, the determination of the NOx storage capacity of the NOx storage catalyst based only on the period of time up to this switch in the sensor signal is subject to errors in such a case. However, this error can be corrected due to the integral detection of the NOx that is desorbed in unreduced form by means of the first signal of the second exhaust gas sensor. The accuracy of the NOx storage capacity of the NOx storage catalyst and/or of the second storage capacity total value thereby ascertained is therefore of interest.
It is thus provided according to the invention that, in the lean and rich operating modes, the internal combustion engine is operated outside of diagnostic operation in a switch from alternating operating phases, wherein a respective phase in the rich operating mode is then terminated at the latest when the amount of NH3 stored in the NH3-SCR storage catalyst has reached a predefinable threshold value, which is lower than the NH3 storage capacity of the NH3-SCR catalyst. This prevents any leakage of NH3 from occurring on the NH3-SCR catalyst during the rich operating mode; that would cause unwanted formation of nitrous oxide due to a reduction of the NH3 reaching the downstream NOx storage catalyst, if it is found that the amount of NH3 stored in the NH3-SCR catalyst has reached the threshold value, then it is preferably provided that the regeneration of the NOx storage catalyst is terminated by switching to lean operation when it still has some NOx stored in it and thus has not yet completely regenerated.
Additional advantages, features and details of the invention are derived from the following description of preferred exemplary embodiments as well as on the basis of the drawings. The features and combinations of features mentioned in the description above as well as those mentioned below in the descriptions of the figures and/or the features and combinations of features shown only in the figures can be used not only in the respective combination indicated but also in other combinations or alone without going, beyond the scope of the invention.